Signal receiver and method for estimating residual doppler frequencies of signals thereof

ABSTRACT

The invention provides a signal receiver. The signal receiver comprises a carrier removal module, a Fast Fourier Transformation (FFT) module, and a signal processor. The carrier removal module generates a carrier signal with a frequency of an estimated carrier frequency, generates at least one delta carrier signal with a frequency of the carrier frequency plus a delta frequency, removes the carrier signal from a first signal to obtain a second signal, and removes the delta carrier signal from the first signal to obtain a third signal. The FFT module derives a series of first FFT values from the second signal, and derives a series of second FFT values from the third signal. The signal processor estimates the carrier frequency of the second signal to obtain the estimated carrier frequency, determines the delta frequency for the carrier removal module, estimates a residual Doppler frequency of the second signal according to both the first FFT values and the second FFT values, and adjusts the estimated carrier frequency according to the residual Doppler frequency.

CROSS REFERENCE

This application is a Continuation of U.S. patent application Ser. No.11/829,231, filed Jul. 27, 2007, now U.S. Pat. No. 7,899,126, and thesubject matter of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to signal processing, and more particularly toFast Fourier Transformation (FFT) of signals.

2. Description of the Related Art

When a Global Positioning System (GPS) receiver receives a GPS signal,the GPS signal is first amplified and filtered. The GPS receiver thenattempts to estimate a Doppler frequency shift of the GPS signal with acarrier tracking loop, and lock a code phase of the GPS signal with acode tracking loop. If the satellite sending the GPS signal is moving,the motion of the satellite causes a Doppler frequency shift, which mustbe adequately compensated before data is extracted from the GPS signal.Additionally, because the satellite combines data carried by the GPSsignal with a pseudo random noise (PRN) code before signal transmission,the GPS receiver must track the code phase of the PRN code of thesatellite to extract data from the GPS signal.

In one method for estimating the residual Doppler frequency of areceived signal, after a carrier signal is removed from a GPS signal,the GPS signal is first delivered to a correlation module which removesthe PRN code from the GPS signal. An FFT module then performs a FastFourier Transformation (FFT) on the GPS signal to obtain a series of FFTvalues, according to which the residual Doppler frequency is estimated.Because the estimation is based on FFT values derived from a GPS signalsubsequent to correlation processing, the estimation method is referredto as “post-correlation FFT” estimation.

FIG. 1 is a block diagram of a GPS receiver 100. A radio-frequency GPSsignal is received by an antenna 102 of the GPS receiver 100. The GPSsignal is then amplified by the pre-amplifier 104. A down converter 106then implements a down conversion on the GPS signal, and an analog todigital converter then converts the GPS signal from analog to digital.The digital GPS signals S_(ch1)˜S_(chN), each of which corresponds to asatellite channel transmitting signal, are then respectively deliveredto corresponding channel processing modules 111˜11N to implement thecarrier tracking loop and code tracking loop, and signalsS_(ch1)′˜S_(chN)′, without carrier and PRN code, are obtained. Thesignals S_(ch1)′˜S_(chN)′ are then delivered to a signal processor 120for extraction of data therefrom.

FIG. 2 is a block diagram of a code removal module 200 of a channelprocessing module. The channel processing module processes a GPS signalS of a specific satellite channel, and the code removal module 200removes the PRN code from the GPS signal S. A sine table 202 and acosine table 204 respectively generate sine values and cosine valuesaccording to a carrier number-controlled oscillator 242. The signal S isthen multiplied by the sine values and the cosine values to respectivelyobtain an in-phase signal I and a quadrature signal Q.

The in-phase signal I and the quadrature signal Q are then delivered toa correlation module 210 of the code removal module 200. A PRN codegenerator 238 first generates three PRN code signals with the same phasedifference therebetween, including an early code E, a prompt code P, anda Late code L. A plurality of multipliers 212˜216 and 222˜226 of thecorrelation module 210 then respectively multiply the in-phase signal Iand the quadrature signal Q by the early code E, the prompt code P, andthe Late code L to obtain a plurality of products. The products are thendelivered to a summation module 220 of the code removal module 200. Aplurality of accumulators 231˜236 of the summation module 220 repeatedlyaccumulate the corresponding products for a predetermined samplingperiod to obtain a plurality of series of samples IE, IP, IL, QE, QP,and QL. A dump counter 240 triggers the accumulators 231˜236 to dump theaccumulation values therefrom.

FIG. 3 is a schematic diagram of a signal processing course ofpost-correlation FFT estimation. As previously mentioned, before a FFTis performed on a received signal to obtain FFT values for estimation, asignal carrier carrying data of the signal and a PRN code mixed with thedata must be removed from the signal. Thus, a signal S₀ is firstdelivered to a carrier removal module 302 for removal of the signalcarrier therefrom. An oscillator 332 generates a carrier signal A withan estimated carrier frequency f_(carrier). A phase rotator 313 rotatesthe phase of the carrier signal A by 90° to obtain a carrier signal A′.Two multipliers 312 and 322 then respectively multiply the signal S₀ bythe carrier signals A and A′ to obtain signals S_(1I) and S_(1Q). Twolow pass filters 314 and 324 then respectively remove high-frequencysignal components from the signals S_(u) and S_(1Q) to obtain signalsS₂₁ and S_(2Q) without signal carriers.

The signals S_(2I) and S_(2Q) are then delivered to a code removalmodule 304 removing a PRN code from the signals. The code removal module304 has a similar structure to the code removal module 200 of FIG. 2. Acode generator 334 generates a PRN code B, and two multipliers 316 and326 of a correlation module multiply the signals S_(2I) and S_(2Q) bythe PRN code B to obtain signals S_(3I) and S_(3Q) without PRN codes.Two integration and dump modules 318 and 328 then repeatedly integratethe signals S_(3I) and S_(3Q) for a duration T and dump the integrationvalues Y_(I) and Y_(Q) to buffers 319 and 329. The buffers 319 and 329temporarily hold the integration values Y_(I) and Y_(Q).

The integration values Y_(I) and Y_(Q) are used as input samples of aFFT module 308. Each corresponding pair of the integration values Y_(I)and Y_(Q) forms a FFT input sample Y with a real part Y_(I) and animaginary part Y_(Q). Because the integration period is T, the samplingfrequency of the FFT input samples Y is 1/T. An FFT point number of theFFT module 308 is assumed to be M. Thus, after the FFT module 308performs a FFT on samples Y₀˜Y_(M-1) of time domain, a plurality of FFTvalues Z₀˜Z_(M-1) of frequency domain are obtained. A signal processorcan then estimate a residual Doppler frequency according to the FFTvalues Z₀˜Z_(M-1).

Because the locally estimated carrier frequency f_(carrier) is notidentical to a true carrier frequency of a satellite transmitting thesignal, the residual Doppler frequency, which is the difference betweenthe locally estimated carrier frequency f_(carrier) and the true carrierfrequency, causes signal distortion and affects data extraction andprocessing, referred to as a residual Doppler effect. The residualDoppler effect can be eliminated by estimating the residual Dopplerfrequency and adjusting the estimated carrier frequency according to theestimated residual Doppler frequency. Because the signal carrier and thePRN code have been removed from the signals S_(3I) and S_(3Q), theresidual Doppler frequency takes effect in the form of a continuoussinusoidal wave mixed in the signals S_(3I) and S_(3Q) with a frequencyof the residual Doppler frequency. Because only a finite segment of thesignal S₃ is sampled as the input samples of the FFT module 308, thecontinuous sinusoidal wave with the residual Doppler frequency isconverted into a sinc function shown in a signal spectrum diagram of theFFT values Z derived from the signal S₃, wherein the center frequency ofthe sinc function is the residual Doppler frequency. Thus, the residualDoppler frequency can be estimated according to the FFT values Z.

FIG. 4A is a signal spectrum diagram of FFT values for residual Dopplerfrequency estimation. A dotted line shows signal degradation of residualDoppler effect. A solid line shows signal spectrum of the continuoussinusoidal wave with a frequency of a residual Doppler frequency, whichis assumed to be 250 Hz in FIG. 4A. A time domain signal is convertedinto FFT input samples with a sampling frequency of 1000 Hz. A FFTmodule then performs FFT on the samples with a FFT point number of 20 togenerate FFT values. Thus, the spectrum range of the FFT values is 1000Hz, ranging from −500 Hz to 500 Hz. 20 FFT values obtained in one FFTconversion are shown in FIG. 4A according to the correspondingfrequencies thereof in the form of circles. Among the 20 FFT values, anFFT value 402 corresponding to the frequency of 250 Hz coincides withthe center frequency of the main lobe of the residual Doppler wave.Thus, in the case of FIG. 4A, the residual Doppler frequency can beeasily estimated by simply locating a corresponding frequency of the FFTvalue 402 with a maximum magnitude among the FFT values.

FIG. 4B is a signal spectrum diagram of FFT values corresponding to ascalloping loss situation. A residual Doppler frequency does not alwayscoincide with the frequency of one of the FFT values. If the frequenciesof FFT values do not exactly match a residual Doppler frequency, thefrequency corresponding to the FFT value with the maximum magnitude doesnot precisely predict the residual Doppler frequency, and scallopingloss occurs. For example, the residual Doppler frequency of FIG. 4B canbe 225 Hz, which lies exactly halfway between the frequencies of the FFTvalues 412 and 414. The frequencies 200 Hz and 250 Hz respectivelycorrespond to the FFT values 412 and 414 with the maximum magnitude, andtherefore are not the exact residual Doppler frequency, differing by afrequency bias of 25 Hz therebetween. Thus, a method is provided toestimate a frequency bias between a frequency of the FFT values and aresidual Doppler frequency.

FIG. 5 is a signal spectrum diagram of FFT values for estimating afrequency bias from scalloping loss. The residual Doppler frequency ofFIG. 5 is 12.5 Hz. Three adjacent FFT values 502, 504, and 506 withmaximum magnitudes are first selected from the 20 FFT values. Thus, thethree adjacent FFT values 502, 504, and 506 form a magnitude peakroughly overlapping the signal spectrum of the main frequency slope ofthe residual Doppler wave, and the residual Doppler frequency can beestimated according to the three FFT values. The estimated residualDoppler frequency can be expressed as the frequency 0 Hz of the peak FFTvalue 504 with the maximum signal magnitude plus a frequency bias of12.5 Hz. Because the frequency bias is substantially in proportion tothe magnitude difference between the left FFT value 502 and the rightFFT value 506, the frequency bias 12.5 Hz can be estimated according tothe difference between the magnitudes 0.18 and 0.3 of the left FFT value502 and the right FFT value 506. The estimated residual Dopplerfrequency is then obtained by adding the frequency of the peak FFT value504 and the estimated frequency bias.

Precision of estimation of the residual Doppler frequency affects theaccuracy of data extraction, further affecting subsequent dataprocessing of the GPS receiver. Because frequency resolution of the FFTmodule is a factor in determining the estimation precision of theresidual Doppler frequency, a higher frequency resolution is desirable.Although the FFT frequency resolution can be increased by increasing FFTpoint numbers of the FFT module, the increase of FFT point numbersextends a filling time of the FFT module, which indicates a periodlength of a segment of GPS signal required by the FFT module to generatea set of FFT values. In other words, there is a tradeoff between thefrequency resolution and the filling time. The increase in filling timecauses the liability of increased signal delay. Thus, a method forimproving a frequency resolution of a FFT without increasing fillingtime is desirable for the estimation of a residual Doppler frequency.

BRIEF SUMMARY OF THE INVENTION

The invention provides a signal receiver. An embodiment of the signalreceiver comprises a carrier removal module, a Fast FourierTransformation (FFT) module, and a signal processor. The carrier removalmodule generates a carrier signal with a frequency of an estimatedcarrier frequency, generates at least one delta carrier signal with afrequency of the carrier frequency plus a delta frequency, removes thecarrier signal from a first signal to obtain a second signal, andremoves the delta carrier signal from the first signal to obtain a thirdsignal. The FFT module derives a series of first FFT values from thesecond signal, and derives a series of second FFT values from the thirdsignal. The signal processor estimates the carrier frequency of thefirst signal to obtain the estimated carrier frequency, determines thedelta frequency for the carrier removal module, estimates a residualDoppler frequency of the second signal according to both the first FFTvalues and the second FFT values, and adjusts the estimated carrierfrequency according to the residual Doppler frequency.

The invention also provides a method for estimating a residual Dopplerfrequency in a signal receiver. First, a carrier frequency of a firstsignal received by the signal receiver is estimated to obtain anestimated carrier frequency. At least one delta frequency is alsodetermined. A carrier signal with a frequency of the estimated carrierfrequency is then generated. At least one delta carrier signal with afrequency of the carrier frequency plus the delta frequency is alsogenerated. The carrier signal is then removed from the first signal toobtain a second signal. The delta carrier signal is then also removedfrom the first signal to obtain a third signal. A first FFT is thenperformed to derive a series of first FFT values from the second signal.A second FFT is also performed to derive a series of second FFT valuesfrom the third signal. A residual Doppler frequency of the second signalis then estimated according to both the first FFT values and the secondFFT values. Finally, the estimated carrier frequency is adjustedaccording to the residual Doppler frequency.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 is a block diagram of a GPS receiver;

FIG. 2 is a block diagram of a code removal module of a channelprocessing module;

FIG. 3 is a schematic diagram of a signal processing course ofpost-correlation FFT estimation;

FIG. 4A is a signal spectrum diagram of FFT values for residual Dopplerfrequency estimation;

FIG. 4B is a signal spectrum diagram of FFT values corresponding to ascalloping loss situation;

FIG. 5 is a signal spectrum diagram of FFT values for estimating afrequency bias from scalloping loss;

FIG. 6 is a block diagram of a signal processor with improved FFTresolution for estimating a residual Doppler frequency according to theinvention;

FIG. 7 is a signal spectrum diagram of FFT values for residual Dopplerfrequency estimation; and

FIG. 8 is a flowchart of a method for estimating a residual Dopplerfrequency with high FFT resolution in a signal receiver according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

FIG. 6 is a block diagram of a signal processor 600 with improved FFTresolution for estimating a residual Doppler frequency according to theinvention. An analog to digital converter 602 first converts an analogsignal S₀ to a digital signal S₁. A code removal module 604 then removesa PRN code from the signal S₁ to obtain a signal S₂, wherein the coderemoval module 604 includes a code generator 606 and a correlationmodule 605 and is substantially similar to the code removal module 200of FIG. 2. The signal S₂ is then delivered to a carrier removal module610. The carrier removal module 610 includes a carrier generator 615generating a carrier signal F with a frequency equal to a carrierfrequency t_(carrier) estimated by a signal processor 620. The carrierremoval module 610 also includes a delta carrier generator 616generating a delta carrier signal F′ with a frequency equal to thecarrier frequency f_(carrier) plus a delta frequency Δf_(carrier).

The delta frequency Δf_(carrier) is lower than a frequency resolution ofa FFT module 626. For example, if N is a FFT point number of the FFTmodule 626, and T is a sampling period of the input samples of the FFTmodule 626, the frequency resolution of the FFT module is 1/(N×T), andthe delta frequency Δf_(carrier) must be lower than the frequencyresolution 1/(N×T). In one embodiment, a plurality of delta frequencieshave different values of J/[(k+1)×(N×T)], wherein k is a number of thedelta frequencies and is greater than or equal to 1, and J is an indexof the delta frequencies and ranges from 1 to k. Thus, if there is onlyone delta frequency, k is equal to 1, and the delta frequencyΔf_(carrier) is 1/[2×(N×T)], and the delta carrier generator 616generates a delta carrier signal F′ with a frequency equal to thecarrier frequency f_(carrier) plus the delta frequency of 1/[2×(N×T)].Two multipliers 611 and 612 then respectively remove the carrier signalF and the delta carrier signal F′ from the signal S₂ to respectivelyobtain signals S₃ and S₄.

A summation module 622 then respectively accumulates samples of thesignals S₃ and S₄. When a duration equal to the sampling period T isexpired, the accumulation values of the signals S₃ and S₄ are output asinput samples of the FFT module 626, and two series of the FFT inputsamples S₅ and S₆ are thus obtained and stored in the buffer module 624.The FFT module 626 then respectively performs FFT on the FFT inputsamples S₅ and S₆ to obtain two sets of FFT values S₇ and S₈respectively corresponding to the signals S₃ and S₄. The coherent memory627 is used to store the coherent accumulated FFT values, which arecomplex numbers. For example, a coherent accumulation period of longerthan 20 ms can be used if the data bit stream is known. The incoherentmemory 628 is used to accumulate the magnitude of the FFT values or thecoherent memory.

The signal processor 620 can then estimate a residual Doppler frequencywith improved FFT resolution according to the accumulated magnitude ofthe FFT values S₇ and S₈. The signal processor 620 first permutes theFFT values S₇ and S₈ according to a frequency order thereof to obtain aseries of interlaced FFT values. Because the FFT values S7 and S₈ arerespectively derived from the signals S3 and S4 obtained by removing thecarrier signal F and the delta carrier signal F′ with a frequencydifference of the delta frequency Δf_(carrier) therebetween, the FFTvalues S₇ and S₈ shown in the same signal spectrum diagram areinterlaced with the frequency interval of the delta frequencyΔf_(carrier). FIG. 7 is a signal spectrum diagram of FFT values forresidual Doppler frequency estimation. The FFT values S₇ derived fromthe signal S₃ are marked with circles in FIG. 7, and the FFT values S₈derived from the signal S₄ are marked with triangles. FIG. 7 clearlyshows that the FFT values S₇ and S₈ are interlaced with a frequencyinterval of 1/[2×(N×T)]. Compared with FIGS. 4A, 4B, and 5 in which onlyone series of FFT values are shown with a frequency resolution of1/[(N×T)], the frequency resolution of the interlaced FFT values is1/[2×(N×T)], significantly improved.

The signal processor 620 can then estimate the residual Dopplerfrequency according to the interlaced FFT values with higher frequencyresolution, further improving precision of the residual Dopplerfrequency estimation. The signal processor 620 estimates the residualDoppler frequency according to the method in FIGS. 4A, 4B, and 5. Thesignal processor 620 first selects a peak FFT value 704 with a maximummagnitude from the interlaced FFT values. Because the magnitudedifference between the left FFT value 708 and the right FFT value 710 iszero, there is no frequency bias from scalloping loss, and the frequency0 Hz corresponding to the peak FFT value 704 directly determines theresidual Doppler frequency. After the residual Doppler frequency of 0 Hzis determined, the signal processor 620 adjusts the estimated carrierfrequency f_(carrier) according to the estimated residual Dopplerfrequency. The carrier NCO 618 of the carrier removal module 610 canthen generate a carrier signal with precise carrier frequency, and thereis no residual Doppler effect causing distortion of signal S₃. Thus, thesignal processor 620 can implement precise data extraction according toa signal without residual Doppler effects.

FIG. 8 is a flowchart of a method 800 for estimating a residual Dopplerfrequency with high FFT resolution in a signal receiver according to theinvention. First, a carrier frequency of a first signal is estimated instep 802. A carrier signal with a frequency equal to the carrierfrequency is then generated in step 804. A delta carrier signal with afrequency equal to the carrier frequency plus a delta frequency is thengenerated in step 806, wherein the delta frequency is lower than afrequency resolution of a first FFT and a second FFT. The first signalis then multiplied by the carrier signal to obtain a second signal instep 808. The first signal is also multiplied by the delta carriersignal to obtain a third signal in step 810. A first FFT is thenperformed on samples derived from the second signal to obtain a seriesof first FFT values in step 812. A second FFT is then performed onsamples derived from the third signal to obtain a series of second FFTvalues in step 814. A residual Doppler frequency is then estimatedaccording to both the first FFT values and the second FFT values in step816. Finally, the carrier signal is adjusted according to the residualDoppler frequency in step 818 to eliminate distortion from residualDoppler effect from the second signal.

The invention provides a signal receiver estimating a residual Dopplerfrequency with high FFT resolution. Two carrier signals respectivelywith the frequencies of a carrier frequency and the carrier frequencyplus a delta frequency are generated, wherein the delta frequency isless than a frequency resolution of a FFT module. The two carriersignals are then respectively removed from a received signal to obtaintwo FFT input signals. The FFT module then performs FFT on both the twoFFT input signals to obtain two series of FFT values, which are arrangedin frequency order to obtain a series of interlaced FFT values withimproved FFT resolution. A residual Doppler frequency can then beprecisely estimated according to the interlaced FFT values. Because FFTresolution is improved, estimation errors from scalloping loss arereduced. The improved frequency resolution also improves precision offrequency jump, multi-path effect, and signal jamming detection.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

What is claimed is:
 1. A signal receiver, comprising: a carrier removalmodule, generating a carrier signal with a frequency of an estimatedcarrier frequency, generating at least one delta carrier signal with afrequency of the estimated carrier frequency plus a delta frequency,removing the carrier signal from a first signal to obtain a secondsignal, and removing the delta carrier signal from the first signal toobtain a third signal; a Fast Fourier Transformation (FFT) module,coupled to the carrier removal module, deriving a series of first FFTvalues from the second signal, and deriving a series of second FFTvalues from the third signal; and a signal processor, coupled to thecarrier removal module and the FFT module, estimating the carrierfrequency of the first signal to obtain the estimated carrier frequency,determining the delta frequency for the carrier removal module,estimating a residual Doppler frequency of the second signal accordingto both the first FFT values and the second FFT values, and adjustingthe estimated carrier frequency according to the residual Dopplerfrequency.
 2. The signal receiver as claimed in claim 1, wherein thefrequency resolution of the FFT module is 1/(N×T), wherein N is a FFTpoint number of the FFT module, and T is a sampling period of the FFTmodule.
 3. The signal receiver as claimed in claim 2, wherein the deltafrequency equals J/[(k+1)×(N×T)], wherein k is a number of the deltacarrier signal and is greater than or equal to 1, and J is an index ofthe delta carrier signal and ranges from 1 to k.
 4. The signal receiveras claimed in claim 1, wherein the carrier removal module comprises: acarrier generator, generating the carrier signal according to theestimated carrier frequency; a delta carrier generator, generating thedelta carrier signal according to the delta frequency; a firstmultiplier, coupled to the carrier generator, multiplying the firstsignal by the carrier signal to obtain the second signal; and a secondmultiplier, coupled to the delta carrier generator, multiplying thefirst signal by the delta carrier signal to obtain the third signal. 5.The signal receiver as claimed in claim 1, wherein a series of first FFTinput samples with the sampling period are derived from the secondsignal, a series of second FFT input samples with the sampling periodare derived from the third signal, and the FFT module then performs aFFT with the FFT point number on the first FFT input samples to obtainthe first FFT values and performs a FFT with the FFT point number on thesecond FFT input samples to obtain the second FFT values.
 6. The signalreceiver as claimed in claim 5, wherein the signal receiver furthercomprises: a summation module, coupled between the carrier removalmodule and the FFT module, repeatedly summing samples of the secondsignal for the duration equal to the sampling period to obtain a seriesof the first FFT input samples, and repeatedly summing samples of thethird signal for the duration equal to the sampling period to obtain aseries of the second FFT input samples; and a buffer module, coupledbetween the summation module and the FFT module, temporarily holding thefirst FFT input samples and the second FFT input samples.
 7. The signalreceiver as claimed in claim 5, wherein the signal receiver furthercomprises: an integration and dump module, coupled between the carrierremoval module and the FFT module, repeatedly integrating the secondsignal for the duration of the sampling period to obtain a series of thefirst FFT input samples, and repeatedly integrating the third signal forthe duration of the sampling period to obtain a series of the second FFTinput samples; and a buffer module, coupled between the integration anddump module and the FFT module, temporarily holding the first FFT inputsamples and the second FFT input samples.
 8. The signal receiver asclaimed in claim 1, wherein the signal processor permutes the first FFTvalues and the second FFT values according to a frequency order thereofto obtain a series of interlaced FFT values with a high FFT frequencyresolution, selects a peak FFT value with a maximum magnitude from theinterlaced FFT values, and estimates the residual Doppler frequencyaccording to the frequency of the peak FFT value, thus the residualDoppler frequency is estimated.
 9. The signal receiver as claimed inclaim 8, wherein the signal processor further selects three adjacent FFTvalues with maximum magnitudes, including a left FFT value with a lowerfrequency, the peak FFT value with a middle frequency, and a right FFTvalue with a higher frequency, from the interlaced FFT values, and thesignal processor then estimates a frequency bias due to scalloping lossaccording to a magnitude difference between the right FFT value and theleft FFT value, and then adds the frequency bias to the middle frequencyof the peak FFT value to obtain the residual Doppler frequency, therebyestimating the residual Doppler frequency.
 10. The signal receiver asclaimed in claim 8, wherein the signal processor interlaces the firstFFT values and the second FFT values to form the interlaced FFT values,thereby increasing a FFT frequency resolution of the interlaced FFTvalues.
 11. The signal receiver as claimed in claim 1, wherein thesignal processor determines a plurality of delta frequencies withdifferent values for the carrier removal module, the carrier removalmodule generates a plurality of delta carrier signals according to thedelta frequencies and removes the delta carrier signals from the firstsignal to obtain a plurality of third signals, the FFT module derives aplurality of series of second FFT values from the third signals, and thesignal processor estimates the residual Doppler frequency according tothe first FFT values and the plurality of second FFT values.
 12. Amethod for estimating a residual Doppler frequency in a signal receiver,comprising: estimating a carrier frequency of the second signal toobtain an estimated carrier frequency; determining at least one deltafrequency; generating a carrier signal with a frequency of the estimatedcarrier frequency; generating at least one delta carrier signal with afrequency of the estimated carrier frequency plus the delta frequency;removing the carrier signal from a first signal received by the signalreceiver to obtain a second signal; removing the delta carrier signalfrom the first signal to obtain a third signal; performing a first FFTto derive a series of first FFT values from the third signal; performinga second FFT to derive a series of second FFT values from the fourthsignal; estimating a residual Doppler frequency of the second signalaccording to both the first FFT values and the second FFT values; andadjusting the estimated carrier frequency according to the residualDoppler frequency.
 13. The method as claimed in claim 12, wherein an FFTmodule of the signal receiver performs the first FFT and the second FFT,and the frequency resolution of the first FFT and the second FFT is1/(N×T), wherein N is a FFT point number of the FFT module, and T is asampling period of the FFT module.
 14. The method as claimed in claim13, wherein the delta frequency equals J/[(k+1)×(N×T)], wherein k is anumber of the delta carrier signal and is greater than or equal to 1,and J is an index of the delta carrier signal and ranges from 1 to(k+1).
 15. The method as claimed in claim 13, wherein the performance ofthe first FFT comprises: deriving a series of first FFT input sampleswith the sampling period from the second signal; and performing thefirst FFT with the FFT point number on the first FFT input samples toobtain the first FFT values; and the performance of the second FFTcomprises: deriving a series of second FFT input samples with thesampling period from the third signal; and performing the second FFTwith the FFT point number on the second FFT input samples to obtain thesecond FFT values.
 16. The method as claimed in claim 15, wherein thefirst FFT input samples are obtained by repeatedly summing samples ofthe second signal for the duration equal to the sampling period, and thesecond FFT input samples are obtained by repeatedly summing samples ofthe third signal for the duration equal to the sampling period.
 17. Themethod as claimed in claim 15, wherein the first FFT input samples areobtained by repeatedly integrating the second signal for the duration ofthe sampling period, and the second FFT input samples are obtained byrepeatedly integrating the third signal for the duration of the samplingperiod.
 18. The method as claimed in claim 12, wherein the removal ofthe carrier signal comprises multiplying the first signal by the carriersignal to obtain the second signal, and the removal of the delta carriersignal comprises multiplying the first signal by the delta carriersignal to obtain the third signal.
 19. The method as claimed in claim12, wherein the estimation of the residual Doppler frequency comprises:permuting the first FFT values and the second FFT values according to afrequency order thereof to obtain a series of interlaced FFT values witha high FFT frequency resolution; selecting a peak FFT value with amaximum magnitude from the interlaced FFT values; and estimating theresidual Doppler frequency according to the frequency of the peak FFTvalue.
 20. The method as claimed in claim 19, wherein the estimation ofthe residual Doppler frequency further comprises: selecting threeadjacent FFT values with maximum magnitudes, including a left FFT valuewith a lower frequency, the peak FFT value with a middle frequency, anda right FFT value with a higher frequency, from the interlaced FFTvalues; estimating a frequency bias from scalloping loss according to amagnitude difference between the right FFT value and the left FFT value;and adding the frequency bias to the middle frequency of the peak FFTvalue to obtain the residual Doppler frequency.
 21. The method asclaimed in claim 19, wherein the permutation of the first FFT values andthe second FFT values is achieved by interlacing the first FFT valuesand the second FFT values to form the interlaced FFT values, therebyincreasing a FFT frequency resolution of the interlaced FFT values.